1. Field of the Invention
The invention relates to an EMUS transducer system and a method for producing linearly polarized transverse waves with a variably predeterminable polarisation direction within a test specimen containing at least ferromagnetic material portions. A test specimen surface with a magnetization unit provided which is arrangeable on the surface of the test specimen and produces a magnetic field orientated parallel to the test specimen surface within the test specimen. At least one HF coil arrangement is provided and is arrangeable on the test specimen surface and for producing or detecting a HF field overlaid with the magnetic field.
2. Description of the Prior Art
Linearly polarized ultrasound (US) transverse waves are preferably used for non-destructive material testing and also for assessing material states, such as textures or inner stresses. To this end, it is necessary to orientate the polarization direction of the linearly polarized US transverse waves to be coupled into the material to be investigated in each case parallel and perpendicularly to the preferential direction of the respective texture or inner stress, for detecting and correspondingly evaluating propagation differences of the US waves characteristic of anisotropic material in respective propagation directions. In addition, by suitable orientation, that is to say rotation of the polarization direction of the US waves relatively to the material structure, for example workpieces made from cast iron, optimum propagation conditions for US waves can be created, in order to bring about evaluable interactions between the US waves and the respective workpiece. It is also possible to detect the orientation of cracks running perpendicularly to the workpiece surface into the interior of the material in a spatially resolved manner.
Linearly polarized US transverse waves are typically produced by piezoelectric or electromagnetic ultrasound transducers.
The use of piezoelectric ultrasound transducers necessitates the use of a coupling means, providing ultrasound signals or vibrations produced by a piezoelectric ultrasound probe to be coupled into a workpiece to be investigated or out of the same again. However, the coupling means, as well as the coupling gap thickness between the workpiece surface and the ultrasound probe, influence the signal quality and therefore the validity of the measurement results in a disadvantageous manner. Especially in the case of transverse waves, there is furthermore a requirement for a coupling medium, which is as highly-viscous as possible, as a result of which the test conditions are made more difficult, particularly in the case of probe movements. In addition, the coupling means contaminates the workpiece surface which is a circumstance occurring undesirably and in particular in the case of investigations on quality surfaces. Additionally, the use of coupling means limits the use of this investigation method on workpieces at higher temperatures.
An alternative to this result is the use of electromagnetic ultrasound transducers (EMUS), which was transversely polarized ultrasound waves guided in a mode-pure manner which can be excited particularly well. In this technology, ultrasound signals are excited and picked up directly in the layers of a component close to the surface without coupling means, that is to say contactlessly by means of electromagnetic interaction. In ferromagnetic materials, the excitation is mainly based on the effect of magnetostriction, by contrast the Lorentz force acts as the excitation mechanism in non-magnetic, electrically conductive materials. More information on this can be drawn from the following articles: Hirao, M. and Ogi, H. (2003), EMATS for Science and Industry (Kluwer Academic Publishers) and also Igarashi, B., Alers, G. A., “Excitation of Bulk Shear Waves in Steel by Magnetostrictive Coupling”, IEEE Ultrasonic Symposium Proceedings (1998) 893-896.
In FIG. 2, a prior art EMUS transducer arrangement is illustrated which produces transversely polarized ultrasound waves within a workpiece 2 made of ferromagnetic material is illustrated. The EMUS transducer has a magnetization unit 1 and also a HF coil system 3. The magnetization unit 1, which can be constructed in the form of a permanent magnet or an electromagnet operated with direct current or low-frequency alternating current, provides two regions 11 and 12 which can be placed onto the test sample surface. One region constitutes the magnetic north pole N and the other region constitutes the magnetic south pole S. In this manner, a magnetic field 13 orientated parallel to the test sample surface is introduced into the test specimen 2. The HF coil system 3, which can be a single coil or of a plurality of coils, is located between the magnetic north and south poles of the magnetization unit 1 as close to the surface as possible on the test specimen surface, in order to produce a modulated high-frequency electromagnetic field 13 which is orientated parallel to the test specimen surface, in the region of the parallel orientated magnetic field 13 within the test specimen 2 by corresponding electrical excitation, preferably by means of a powerful HF burst signal. Due to magnetostrictive effects acting within the ferromagnetic material of the test specimen 2, oscillating forces arise due to the modulation of the quasi-stationary magnetic field 13 by the high-frequency electromagnetic field. These oscillating forces are used as sources for ultrasound signals, particularly in the form of transversely polarized ultrasound waves 31 propagating perpendicularly to the test specimen surface. In the case of reception, the processes are reciprocal to these processes. More details about this can be drawn from the following article: Niese, F., Yashan, A., Wilhelms, H., “Wall Thickness Measurement Sensor for Pipeline Inspection Using EMAT Technology in Combination with Pulsed Eddy Current and Magnetic Flux Leakage”, 9th European Conference on NDT, 2006, Berlin.
In order to be able to perform material investigations with an EMUS probe, it is necessary to rotate the entire EMUS probe relative to the test specimen surface, particularly as the polarization direction of a linearly polarized transverse wave 31 created with an EMUS probe of the type of construction illustrated in FIG. 2 is always aligned parallel to the magnetization direction along the parallel running magnetic lines of the magnetic field 13 within the test specimen 2. A rotation at least of the magnetization unit 1 is in many cases connected with additional mechanical outlay which is furthermore undesirable, particularly as the measurement constellation relative to the test object is subject to a change as a result, which is error-prone for an exact investigation of location-dependent material states in test specimens.
A test apparatus for testing ferromagnetic workpieces using ultrasound waves is disclosed in DE 41 01 942, which provides an overlaying of an alternating magnetic field orientated parallel to the workpiece surface with a stationary or quasi-stationary magnetic field orientated perpendicularly to the workpiece surface for the oblique sonic impingement of vertically polarized ultrasound waves into the workpiece to be investigated. A HF coil arrangement for coupling eddy currents into the workpiece is attached in the region of the magnetic field orientated perpendicularly to the workpiece surface. For the direction-selective sonic impingement, high-frequency transmitting pulses are triggered for controlling the HF coil arrangement either in the region of the lower or upper half waves of the alternating current. It is neither possible to create ultrasound waves propagating into the workpiece vertically to the workpiece surface with a test arrangement of this type. Moreover, influence on the spatial orientation of the polarization direction is not provided.